In general, GPS (Global Positioning System) positioning techniques fall into two main categories, navigation and surveying. Traditionally, surveying has required greater precision and thus used differential techniques requiring long periods of observation, whereas navigation has required less precision and emphasized real-time position determination. The development of real-time kinematic (RTK) positioning has resulted in systems that are widely applied to both surveying and navigation.
An RTK positioning system typically includes a constellation of satellites, a base or reference station, and a rover.
For purposes of this disclosure, a satellite refers to any fundamental source of raw position data such as that transmitted by the GPS, GLONASS, or planned Galileo orbiting satellites, or an earthbound source (e.g., pseudolite). A satellite positioning system (SATPS) refers to a system using either extra-terrestrial satellites or terrestrial satellites (pseudolites) as sources of raw position data.
A reference station incorporates a SATPS data receiver and a wireless link for communication with the rover. The reference station may be a single base station, collection of base stations, or a virtual reference station (VRS). More than one rover may be linked to a reference station, particularly when the reference station comprises a network of base stations. Further information regarding virtual reference stations is provided in U.S. patent application Ser. No. 10/078,294, Kirk et al, filed Feb. 15, 2002, which is incorporated herein by reference.
In RTK, centimeter level accuracy is obtained by having both the reference station and the rover track the carrier phases of the same satellite(s) at the same time. High accuracy position determination is achieved by applying a mathematical model for relative positioning (e.g., double-difference model) to the simultaneous measurements of the reference station and rover. The double differences essentially eliminate common mode errors (e.g. clock errors), and can be processed to produce a precise baseline (dx, dy, dz) between the reference station and rover. When the reference station position is accurately known in a given coordinate frame, the rover position can also be determined in the same frame. Even if the reference station position is not accurately known, precise relative positioning can still be carried out.
The data collected by the reference station and the data collected by the rover is combined at a single location for processing and position determination. For RTK, this data combination is usually done by wireless communication, and the data may be combined at the reference station, rover, or other location for processing. It is the immediate communication between the rover and reference station that enables real-time determination of relative or absolute position, velocity and time, as opposed to post processing.
The data communication between a rover and a reference station may be unidirectional or bidirectional, depending upon the allocation of the data processing functions and the type of wireless link employed. Depending upon the RF band used for data transmission, power and distance limitations may result in the rover having only a receiving capability when direct radio communication is used.
An alternative to direct radio communication is the use of a network such as a cellular network or other wireless network. The use of a wireless network can overcome problems with interference, distance, licensing, etc.
Historically, the link between a reference station and a rover has been made as a continuous connection with periodic transmission of data. Correction data from a reference station may be formatted according to various proprietary or published formats, e.g., the Radio Technical Commission Marine (RTCM) format or the Trimble CMR/CMR+. For RTK using double differencing and the RTCM format messages 18 and 19, data is updated about every 0.5 to 2.0 seconds. For RTK, data rates of between 2,400 bps and 9,600 bps are commonly used. The data transmission rate may be manually selectable.
Current RTK systems often have a dedicated wireless link, so there is no economic incentive to reduce the amount of data transmitted. Although the transmission rate may be selected by the user, it is not automatically controlled on the basis of economic considerations. The lack of an incentive to reduce the amount of data transmitted over a direct link contributes to crowding of the frequency band used for transmission and reduces overall efficiency in both spectrum usage and in transmitter power consumption.
Packet-switched services such as Cellular Digital Packet Data (CDPD), General Packet Radio Service (GPRS), Enhanced Data Rates for Global Evolution (EDGE) and 3G Packet Data, and wireless IP networks are examples of wireless communications networks that can be used to provide communications for an RTK positioning system as an alternative to direct radio links. The use of these services can overcome difficulties associated with direct radio communications; however, the cost of these packet switched services can be based upon data throughput, and traditional communications in RTK systems lack a method for evaluating the cost/benefit of transmitted data. When high bandwidth packet-switched networks are used, considerable cost can be incurred when large volumes of data with little or no value are transmitted.
Thus a need exists for a method of controlling the data transmission rate in an RTK positioning system so that cost effective position determination can be performed when using communications networks that have throughput based costs. There is also a need for mitigation of interference in crowded regions of the electromagnetic spectrum used for communications.